In the fabrication of through-hole PCBs (printed circuit boards), there has heretofore been widely used the solder through-hole process in which solder is applied by electroplating. In this method, however, the danger of short-circuiting during soldering with more complex and dense circuit geometry on the PCB is increased. A solder resist, therefore, is often applied in order to avoid this problem. It is preferred that such a solder resist be applied to a copper plate rather than a solder plate since it adheres better on the copper plate than the solder plate. Thus it has been desired to develop a method in which after the through-hole is completed by the application of solder plating the solder is stripped or removed to form a copper through-hole.
Various techniques are known for stripping tin or tin alloys from metallic copper. Typical techniques are described in Japanese Patent Publication No. 17336/75 (Enthone, Inc. (U.S.A.)) and U.S. Pat. Nos. 3,990,982, 3,926,699, and 3,841,905 (RBP Chemical Corp. U.S.A.)).
The technique disclosed in the Enthone, Inc. patent is mainly directed to stripping solder plate at the terminals of a printed circuit board. Although this technique can be applied to the fabrication of a copper through-hole PCB, it has the disadvantage that the thiourea used as an additive in the stripping solution is absorbed onto a copper and deteriorates the adhesion of the solder resist applied at a subsequent step and solderability.
The stripping solutions disclosed in the RBP Corp. patents are mainly composed of acidic ammonium fluoride and hydrogen peroxide with suitable stabilizers. However, generally safety codes require that chemical compositions containing acidic ammonium fluoride, which is highly poisonous, must be specifically handled. A stripping solution containing acidic ammonium fluoride, therefore, is subject to various specific handling regulations and is not convenient to use. Furthermore, such stripping solutions have the disadvantage that they attack glass fibers in the substrate of a printed circuit board, causing measling.